Move time from DataPoint to Sum/Gauge

[fraillt]

Fixes #1566 (partially, because Histograms are not touched by this PR).

Changes

Move time and start_time fields from DataPoint to specific aggregations, - Sum and Gauge.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 77.69784% with 31 lines in your changes missing coverage. Please review.

Project coverage is 79.4%. Comparing base (f513768) to head (2abdf0b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	22 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	4 Missing ⚠️
...-sdk/src/metrics/internal/exponential_histogram.rs	25.0%	3 Missing ⚠️
...etry-sdk/src/testing/metrics/in_memory_exporter.rs	83.3%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2377     +/-   ##
=======================================
+ Coverage   79.3%   79.4%   +0.1%     
=======================================
  Files        123     123             
  Lines      21535   21527      -8     
=======================================
+ Hits       17082   17107     +25     
+ Misses      4453    4420     -33     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cijothomas]

LGTM. Need to fix the stdout exporter before merge.

[cijothomas]

@fraillt Unfortunately, we need to fix conflicts before merging this one.!

[fraillt]

Unfortunately merging was equal to rewriting basically, so I did clean rebase...

[cijothomas]

@fraillt could you send a PR to note all the breaking changes? (We can mention that these affects only those people who write custom Exporters)